Moving data from file on storage volume to alternate location to free space

ABSTRACT

A ghosting filter on a computing device perceives that a file is in a ghosted form and commands a file system of the device to retrieve ghosting information from the ghosted file. The ghosting filter receives a read command from an application on the device with respect to a portion of the data of the ghosted file. The read command has parameters including an offset into the portion of the data and a read length with respect to such portion. The ghosting filter initiates reconstituting of the portion of the data of the ghosted file based on the received ghosting information and data of the ghosted file at an alternate location, and after the portion of the data of the ghosted file has been reconstituted responds to the read command with the portion of the data requested.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application shares a common disclosure with:

U.S. patent application Ser. No. ______, (Attorney Docket No.Msft-5019/313836.1) and entitled “MOVING DATA FROM FILE ON STORAGEVOLUME TO ALTERNATE LOCATION TO FREE SPACE”,

U.S. patent application Ser. No. ______, (Attorney Docket No.Msft-5020/313837.1) and entitled “MOVING DATA FROM FILE ON STORAGEVOLUME TO ALTERNATE LOCATION TO FREE SPACE”,

U.S. patent application Ser. No. ______ (Attorney Docket No.Msft-5021/313838.1) and entitled “MOVING DATA FROM FILE ON STORAGEVOLUME TO ALTERNATE LOCATION TO FREE SPACE”,

U.S. patent application Ser. No. ______ (Attorney Docket No.Msft-5023/313840.1) and entitled “MOVING DATA FROM FILE ON STORAGEVOLUME TO ALTERNATE LOCATION TO FREE SPACE”, and

U.S. patent application Ser. No. ______ (Attorney Docket No.Msft-5053/313951.1) and entitled “MOVING DATA FROM FILE ON STORAGEVOLUME TO ALTERNATE LOCATION TO FREE SPACE”,

all of which are filed concurrently with the present application, andall of which are hereby incorporated by reference in their entirety.

TECHNICAL FIELD

The present invention relates to an architecture and methods that allowdata from a computer file stored on a storage volume to be moved or‘ghosted’ to an alternate location to free space on the storage volume.More particularly, the present invention relates to such an architectureand methods whereby the remainder of the ghosted file stays on thestorage volume and the ghosted data at the alternate location can ifnecessary be retrieved and placed back into the ghosted file to resultin a de-ghosted file.

BACKGROUND OF THE INVENTION

In a computing device such as a personal computer, a computer server, orthe like, and as is known, data is typically persistently stored on thecomputing device in the form of local computer files resident on one ormore local storage volumes of the computing device. Each such storagevolume may reside on a hard drive of the computing device or the like,and such storage volume may be organized by, accessed through, andotherwise controlled by a file system running on the computing device,as is also known.

At times, it can be the case that some, many, and perhaps even most ofthe computer files on the storage volume are not of interest and can beconsidered to have become ‘cold’. That is to say, such cold files havenot been accessed for some extended length of time and/or likely willnot be accessed for some extended length of time, for example, andtherefore have little if any real value in remaining on the volume.

Of course, such cold files could simply be deleted from the storagevolume, especially if there is a need for space on such volume. However,it is to be appreciated that most users are loathe to delete filessimply to create space. In addition, it is considered bad practice todelete a cold file merely because of perceived disuse. At any rate, itcan and likely will be the case that a cold file though not consideredneeded and not foreseen as needed may nevertheless become needed at somefuture point.

In such a situation, then, it would be useful to be able to create spaceon the volume by moving data from such cold files to an alternatelocation, while still allowing such cold files to maintain a presence onthe volume. That is, it would be useful to be able to ‘ghost’ a coldfile by moving data from such cold files to the alternate location or tocopy the entire file to the alternate location, while leaving the coldfile in a smaller, ‘ghosted’ form on the volume. Thus, if in fact theghosted file is needed on the computing device, the data for suchghosted file may be retrieved from the alternate location, the ghostedfile may be reconstituted based on thereon, and such reconstituted filemay then be employed.

In another scenario, it can be the case that data files of anorganization or the like are to be replicated at multiple branchlocations of the organization, perhaps from a central location such as ahub. For example, an architectural design firm may wish to have allarchitectural design files thereof available at any one of severalbranches of such firm.

In such a situation, a networked system can be constructed to replicatea copy of each file to each branch, and to keep all files at allbranches current. In such a system, for example, a centralized hubserver would store each such file, and a replication service would beemployed to distribute a copy of each file at the hub over a network toa branch server or the like at each branch. However, it is to beappreciated that as the number of files of the organization increases,and as the size of each file increases, and as the number of branchesincreases, the amount of traffic over the network also increases,perhaps to a point that exceeds available bandwidth. Moreover, as theoverall size of all the files at the hub server increases, it may infact become the case that each branch server does not have enough spaceavailable thereon to store all of such files as replicated from the hubserver.

Similar to the previous scenario, though, it can be the case that some,many, and perhaps even most of the computer files on the branch serverof a particular branch are not of interest and can be considered to besuperfluous. Such superfluous files may for example relate to mattersthat are not relevant to the particular branch, likely will not beaccessed through the particular branch, and therefore have little if anyreal value in being replicated on the branch server for the particularbranch. For example, a branch office of an architectural design firm inWilkes-Barre, Pa. likely has very little if any need to havearchitectural design files relating to a project handled by a branchoffice of the firm in Boynton Beach, Fla.

In such a situation, then, and similar to previous scenario, it would beuseful to be able to wholly store on a branch server of a particularbranch only those files relevant to the particular branch, while onlypartially storing on such branch server of such particular branch allother non-relevant files as available from the hub server for theorganization. Thus, in a manner akin to that of the previous scenario,it would be useful to be able to ‘ghost’ the non-relevant files at thebranch server of the particular branch, such that each non-relevant fileremains on the branch server in a smaller, ‘ghosted’ form. Thus, andagain, if in fact the ghosted file is needed at the branch server, thedata for such ghosted file may be retrieved from the hub server, theghosted file may be reconstituted based on thereon, and suchreconstituted file may then be employed.

Accordingly, a need exists for a method and mechanism by which a file ata source such as a local volume or a branch server may be replicated orghosted such that the data thereof is stored at a sink such as analternate location or a hub server, and the file at the source is thusin a reduced or ghosted form that can be reconstituted if need be. Inparticular, a need exists for such a method and mechanism by which sucha ghosted file may be formed and reconstituted, as necessary.

SUMMARY OF THE INVENTION

The aforementioned needs are satisfied at least in part by the presentinvention in which a method is provided in connection with a computingdevice having a storage volume, a file system managing the storagevolume, and a file stored on the storage volume by the file system andaccessed by such file system. The file is defined to include data andmetadata relating to the data, but is in a ghosted form where the datahas been moved to an alternate location to free space on the volume. Themetadata of the ghosted file includes ghosting information for beingemployed to at least partially retrieve the moved data for the file fromthe alternate location. The method is performed in response to the filesystem receiving an open command from an application on the computingdevice with respect to the ghosted file, where the file system uponreceiving the open command notes that the ghosted file does not containthe moved data and returns an error.

In the method, a ghosting filter on the computing device intercepts suchreturned error and perceives based thereon that the file is in fact in aghosted form. Thus, the ghosting filter commands the file system toretrieve the ghosting information from the ghosted file and receivessuch ghosting information. The ghosting filter creates a handlecorresponding to the ghosted file, and delivers the handle to theapplication as the response to the open command therefrom, andthereafter the ghosting filter receives a read command from theapplication with respect to a portion of the data of the opened ghostedfile. The read command has parameters including the handle and adefinition of the portion including an offset into the portion of thedata of the ghosted file and a read length with respect to such portionof the data. The ghosting filter initiates reconstituting of the portionof the data of the ghosted file based on the received ghostinginformation and the data of the ghosted file at the alternate location,and after the portion of the data of the ghosted file has beenreconstituted responds to the read command with the portion of the datarequested.

BRIEF DESCRIPTION OF THE DRAWINGS

The foregoing summary, as well as the following detailed description ofthe embodiments of the present invention, will be better understood whenread in conjunction with the appended drawings. For the purpose ofillustrating the invention, there are shown in the drawings embodimentswhich are presently preferred. As should be understood, however, theinvention is not limited to the precise arrangements andinstrumentalities shown. In the drawings:

FIG. 1 is a block diagram representing a general purpose computer systemin which aspects of the present invention and/or portions thereof may beincorporated;

FIG. 2 is a block diagram showing a file ghosted at a source such thatthe data thereof is stored at a sink in accordance with embodiments ofthe present invention;

FIGS. 3A and 3B are block diagrams showing the file and data of FIG. 2,whereby only the data is stored at the sink (FIG. 3A) and whereby theentire file is stored at the sink (FIG. 3B) in accordance withembodiments of the present invention;

FIG. 4 is a flow diagram showing key steps performed in ghosting a fileto the sink of FIG. 2 in accordance with one embodiment of the presentinvention;

FIG. 5 is a flow diagram showing key steps performed in reconstitutingthe ghosted file from the sink of FIG. 2 in accordance with oneembodiment of the present invention;

FIG. 6 is a flow diagram showing in more detail key steps performed inreconstituting the ghosted file from the sink of FIG. 2 in accordancewith one embodiment of the present invention;

FIG. 7 is a flow diagram showing key steps performed in partiallyreconstituting the ghosted file from the sink of FIG. 2 in accordancewith one embodiment of the present invention;

FIG. 8 is a block diagram showing various statuses for the data of thefile of FIG. 2 in accordance with one embodiments of the presentinvention;

FIG. 9 is a flow diagram showing key steps performed in efficientlyreconstituting at least a portion of the ghosted file from the sink ofFIG. 2 based on the status of the segments of the data of the portion asset forth in FIG. 8 in accordance with one embodiment of the presentinvention;

FIG. 10 is a flow diagram showing key steps performed when re-ghostingfiles to the sink of FIG. 2 in accordance with one embodiment of thepresent invention;

FIG. 11 is a block diagram showing the source of FIG. 2 associated witha plurality of sinks, whereby the source has a single generic ghostingfilter and a ghosting manager corresponding to each sink, in accordancewith one embodiment of the present invention; and

FIG. 12 is a flow diagram showing key steps performed by the ghostingfilter of FIG. 11 in ghosting and reconstituting a file at the source inaccordance with one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Computer Environment

FIG. 1 and the following discussion are intended to provide a briefgeneral description of a suitable computing environment in which thepresent invention and/or portions thereof may be implemented. Althoughnot required, the invention is described in the general context ofcomputer-executable instructions, such as program modules, beingexecuted by a computer, such as a client workstation or a server.Generally, program modules include routines, programs, objects,components, data structures and the like that perform particular tasksor implement particular abstract data types. Moreover, it should beappreciated that the invention and/or portions thereof may be practicedwith other computer system configurations, including hand-held devices,multi-processor systems, microprocessor-based or programmable consumerelectronics, network PCs, minicomputers, mainframe computers and thelike. The invention may also be practiced in distributed computingenvironments where tasks are performed by remote processing devices thatare linked through a communications network. In a distributed computingenvironment, program modules may be located in both local and remotememory storage devices.

As shown in FIG. 1, an exemplary general purpose computing systemincludes a conventional personal computer 120 or the like, including aprocessing unit 121, a system memory 122, and a system bus 123 thatcouples various system components including the system memory to theprocessing unit 121. The system bus 123 may be any of several types ofbus structures including a memory bus or memory controller, a peripheralbus, and a local bus using any of a variety of bus architectures. Thesystem memory includes read-only memory (ROM) 124 and random accessmemory (RAM) 125. A basic input/output system 126 (BIOS), containing thebasic routines that help to transfer information between elements withinthe personal computer 120, such as during start-up, is stored in ROM124.

The personal computer 120 may further include a hard disk drive 127 forreading from and writing to a hard disk (not shown), a magnetic diskdrive 128 for reading from or writing to a removable magnetic disk 129,and an optical disk drive 130 for reading from or writing to a removableoptical disk 131 such as a CD-ROM or other optical media. The hard diskdrive 127, magnetic disk drive 128, and optical disk drive 130 areconnected to the system bus 123 by a hard disk drive interface 132, amagnetic disk drive interface 133, and an optical drive interface 134,respectively. The drives and their associated computer-readable mediaprovide non-volatile storage of computer readable instructions, datastructures, program modules and other data for the personal computer120.

Although the exemplary environment described herein employs a hard disk,a removable magnetic disk 129, and a removable optical disk 131, itshould be appreciated that other types of computer readable media whichcan store data that is accessible by a computer may also be used in theexemplary operating environment. Such other types of media include amagnetic cassette, a flash memory card, a digital video disk, aBernoulli cartridge, a random access memory (RAM), a read-only memory(ROM), and the like.

A number of program modules may be stored on the hard disk, magneticdisk 129, optical disk 131, ROM 124 or RAM 125, including an operatingsystem 135, one or more application programs 136, other program modules137 and program data 138. A user may enter commands and information intothe personal computer 120 through input devices such as a keyboard 140and pointing device 142. Other input devices (not shown) may include amicrophone, joystick, game pad, satellite disk, scanner, or the like.These and other input devices are often connected to the processing unit121 through a serial port interface 146 that is coupled to the systembus, but may be connected by other interfaces, such as a parallel port,game port, or universal serial bus (USB). A monitor 147 or other type ofdisplay device is also connected to the system bus 123 via an interface,such as a video adapter 148. In addition to the monitor 147, a personalcomputer typically includes other peripheral output devices (not shown),such as speakers and printers. The exemplary system of FIG. 1 alsoincludes a host adapter 155, a Small Computer System Interface (SCSI)bus 156, and an external storage device 162 connected to the SCSI bus156.

The personal computer 120 may operate in a networked environment usinglogical connections to one or more remote computers, such as a remotecomputer 149. The remote computer 149 may be another personal computer,a server, a router, a network PC, a peer device or other common networknode, and typically includes many or all of the elements described aboverelative to the personal computer 120, although only a memory storagedevice 150 has been illustrated in FIG. 1. The logical connectionsdepicted in FIG. 1 include a local area network (LAN) 151 and a widearea network (WAN) 152. Such networking environments are commonplace inoffices, enterprise-wide computer networks, intranets, and the Internet.

When used in a LAN networking environment, the personal computer 120 isconnected to the LAN 151 through a network interface or adapter 153.When used in a WAN networking environment, the personal computer 120typically includes a modem 154 or other means for establishingcommunications over the wide area network 152, such as the Internet. Themodem 154, which may be internal or external, is connected to the systembus 123 via the serial port interface 146. In a networked environment,program modules depicted relative to the personal computer 120, orportions thereof, may be stored in the remote memory storage device. Itwill be appreciated that the network connections shown are exemplary andother means of establishing a communications link between the computersmay be used.

Ghosting and Reconstituting a File

In the present invention, and turning now to FIG. 2, a computing device10 such as a personal computer or a computer server or the like has astorage volume 12 such as a hard drive or a persistent RAM drive or thelike, the volume 12 has stored thereon a number of computer files 14,and the files 14 on the volume 12 are organized by, accessed through,and otherwise controlled by a file system 16 running on the computingdevice 10. As may be appreciated, the computing device 10, volume 12,files 14, and file system 16 may be any type of computing device,volume, file, and file system without departing from the spirit andscope of the present invention.

In one embodiment of the present invention, the computing device 10 is apersonal computer or the like and at least some of the files 14 on thevolume 12 thereof have been determined to be cold in that such files 14for example have not been accessed for some extended length of timeand/or likely will not be accessed for some extended length of time, andtherefore have little if any real value in remaining on the volume 12.Note, though, that coldness of a file 14 may be defined in anyappropriate manner without departing from the spirit and scope of thepresent invention.

At any rate, upon being determined to be cold, a file 14 is not deletedfrom the volume 12 but instead is reduced in size on the volume 12 byremoving at least some data 20 thereof to an alternate location 18. Suchalternate location 18 may be local to the computing device or may beremote therefrom. In general, such alternate location 18 isappropriately coupled to the computing device 10 and may be anyappropriate storage location without departing from the spirit and scopeof the present invention. For example, the alternate location 18 may beanother volume 12 on the storage device 10, another volume 12 on anothercomputing device 10, a file warehouse at a server, a long-term storagedevice at a remote server, or the like.

With the alternate location 18, then, space on the volume 12 may befreed by moving the aforementioned data 20 from such cold files 14 tosuch alternate location 18. Significantly, although the data 20 fromsuch a cold file 14 has been moved, such cold file 14 remains as apresence or ‘ghost’ on the volume 12, albeit in a reduced or ‘ghosted’form. Thus, if in fact the ghosted file 14 is needed on the computingdevice 10, such ghosted file 14 is reconstituted by retrieving the data20 thereof from the alternate location 18 and re-associating suchretrieved data 20 with such ghosted file 14 to form the reconstitutedfile 14. As may be appreciated, then, once reconstituted, the file 14may then in fact be employed.

In another embodiment of the present invention, the computing device 10is a branch server or the like and at least some of the files 14 on thevolume 12 thereof have been determined to be irrelevant in that suchfiles 14 for example are not relevant to a branch office associated withsuch branch server. Of course, irrelevance with respect to a file 14 maybe defined in any appropriate manner without departing from the spiritand scope of the present invention.

At any rate, and as before, an irrelevant file 14 is maintained on thevolume 12 of the branch server 10 in a reduced format without at leastsome data 20 thereof. Here such data 20 is stored at an alternatelocation 18 such as a centralized hub server as maintained by anorganization of which the branch office is a branch. Again, such hubserver 18 is appropriately coupled to the branch server 10.

With the hub server 18, then, space used on the volume 12 of the branchserver 10 may be minimized, and bandwidth necessary to populate suchspace may also be minimized, by storing such aforementioned data 20 fromsuch irrelevant files 14 at such hub server 18. Significantly, andsimilar to before, although the data 20 from such an irrelevant file 14is not present at the branch server 10, such irrelevant file 14 remainsas a presence or ‘ghost’ on the volume 12 of the branch server, albeitin a reduced or ‘ghosted’ form. Thus, if in fact the ghosted file 14 isneeded at the branch server 10, such ghosted file 14 is reconstituted byretrieving the data 20 thereof from the hub server 18 and re-associatingsuch retrieved data 20 with such ghosted file 14 to form thereconstituted file 14. Again, once reconstituted, the file 14 may thenin fact be employed.

To generalize then, whether in an arrangement involving a computingdevice 10 such as a personal computer and an alternate location 18, abranch server 10 and a hub server 18, or otherwise, ‘ghosting’ involvesa file 14 on a volume 12 at a source 10, as seen in FIG. 2, where adetermination has been made that the file 14 should only be present onthe volume 12 in a reduced format that does not include some portion ofthe data 20 associated with such file 14. Accordingly, and as seen inFIG. 2, such data 20 is stored at a sink 18 appropriately coupled to thesource 10.

With such sink 18, then, space used on the volume 12 of the source 10may be minimized in that a ghosted file 14 on the volume 12 is presentin a reduced or ‘stub’ form. Thus, if in fact the ghosted file 14 isneeded at the source 10, such ghosted file 14 is reconstituted byretrieving the data 20 thereof from the sink 18 and re-associating suchretrieved data 20 with such ghosted file 14 to form the reconstitutedfile 14. Once again, once reconstituted, the file 14 may then in fact beemployed at the source 10 as necessary.

In one embodiment of the present invention, the stub of a ghosted file14 as present on the volume of a source 10 is stored in the samelocation on the volume 12 as the original file 14. Thus, whether thefile system 16 organizes the volume 12 according to a directory formator otherwise, the file system 16 looking in a particular location for afile 14 that has been ghosted should find the stub for the file 14,i.e., the ghosted file 14, at such location. Significantly, the ghostedfile 14 as resident on the volume 12 of the source 10 contains allmetadata 22 from the original, non-ghosted file 14, and also containsghosting information 24 that may be employed to retrieve the data 20 forthe file 14 from the sink 18, among other things. As may be appreciated,such ghosting information 24 may be added to the metadata 22 for thefile 14 or may be stored in another location of the file 14.

Accordingly, a user or the like can browse the entire volume 12 of thesource 10 by way of the file system 16 even if some of the files 14 onthe volume 12 are ghosted and some of the files 14 on the volume 12 arenot ghosted. In particular, the metadata 22 associated with a ghostedfile 14 may be employed during such browsing to, among other things,identify the ghosted file 14, provide size information, dateinformation, and the like. When the user tries to access a ghosted file14, the data 20 thereof is retrieved based on the ghosting information24 present in the ghosted file 14, the ghosted file 14 is reconstitutedinto a non-ghosted file 14, and the non-ghosted file 14 may then in factbe accessed by the user.

Turning now to FIG. 3A, it is seen that a non-ghosted file 14 as may beresident on a volume 12 at the source 10 includes metadata 22 that maybe organized into a header or the like, where such metadata 22 includesinformation relating to the data 20 of the un-ghosted file 14 such as alogical file size, a size on the volume 12, a create time, a modifytime, an access time, access permissions, and various file attributes.In addition, and as should be evident, such non-ghosted file 14 asresident on the volume 12 at the source 10 includes the actual data 20of the file 14. Note here that such actual data 20 may be organized asprimary data and alternate data, where the primary data is the data 20from the file 14 as may be employed by an application 30 or the like(FIG. 2), while the alternate data is the data from the file 14 that hasbeen created for other uses. As but one example, such alternate data mayinclude a graphic representation such as a ‘thumbnail’ that can beemployed when displaying a representation of the file 14, perhaps by theapplication 30 or the file system 16 at the source 10.

At any rate, it is likely the case that the primary data is the bulk ofthe data 20 by size. Accordingly, when the non-ghosted file 14 is infact ghosted, it may in fact be the case that only the primary data ofthe data 20 is removed from the remainder of non-ghosted file 14 to formthe ghosted file 14, and that only such primary data is stored at thesink 18, perhaps along with an identification of the ghosted file 14, asis shown in FIG. 3A. Of course, other portions of the file 14 may alsobe removed during ghosting of a file 14 without departing from thespirit and scope of the present invention.

As may be appreciated, though, when the non-ghosted file 14 is in factghosted in the context of a branch server 10 and a hub server 18, wherethe hub server 18 replicates files 14 to the branch server 10, it may infact be the case that while the primary data of the data 20 is removedfrom the remainder of the replicated file 14 at the source 18 to formthe ghosted file 14, the entirety of such file 14 is stored at the sink18, perhaps along with an identification of the ghosted file 14, as isshown in FIG. 3B.

To convert a non-ghosted file 14 to a ghosted file 14 (i.e., to ghost afile 14), and referring now to FIGS. 3 and 4, the file 14 is firstidentified (step 401), and thereafter the data 20 to be removed from thefile 14 is identified (step 403). Again, such data 20 is typically theprimary data of the data 20, but could be other data from such data 20.At any rate, the identified data 20 is then moved from the source 10 tothe sink 18 by way of an appropriate transport mechanism and conduit(step 405), and such moved data 20 is then stored at the sink 18 in anappropriate format (step 407). As may be appreciated, storing the data20 at the sink 18 may be done in any appropriate format and mannerwithout departing from the spirit and scope of the present invention.For example, the data 20 may be compressed and/or encrypted if sodesired, and such data 20 should be stored in a manner so that such data20 can be retrieved with relative ease upon a request to reconstitutethe file 14. For example, such data 20 may be stored according to an IDemployed for retrieval, as will be set forth in more detail below.

Once the data 20 has been moved as at step 405, the space occupied bysuch data 20 on the volume 12 of the source 10 need not remain allocatedto the file 14, as may be appreciated. Accordingly, such allocated spacecan be marked as free (step 409), with the result being that thenow-ghosted file 14 is marked as being sparse. Significantly, althoughsuch allocated space has been freed from use by the ghosted file 14, thelogical file size of the now-ghosted file 14 should not be changed inthe metadata 22 for such file 14 or elsewhere, although the size onvolume 12 for the file 14 may in fact be changed to reflect such freedspace.

In addition, in one embodiment of the present invention, the metadata 22for the now-ghosted file 14 is amended to show that such file 14 is nowghosted (step 411). To do so, for example, a ‘ghosted’ attribute may beset in the metadata 22. As may be appreciated, such set ghostedattribute may be employed primarily as a signal to an inquiringapplication 30 or the like that the file 14 is in fact ghosted. Withsuch a signal, then, the inquiring application 30 can appreciate notonly that the file 14 is in fact ghosted but that accessing such file 14involves a cost in that such file 14 must first be reconstituted into anon-ghosted form. As should be understood, such cost may be in terms ofbandwidth needed to access the data 20 for the file 14 from the sink 18,space required for the data 20 on the volume 12, and/or latency toaccess the data 20 at the sink 18 and reconstitute the file 14 basedthereon, among other things.

Also, in one embodiment of the present invention, the metadata 22 forthe now-ghosted file 14 is amended to include the aforementionedghosting information 24 (step 413). Again, such ghosting information 24includes information that may be employed to retrieve the data 20 forthe file 14 from the sink 18, among other things. For example, suchghosting information 24 may include the ID stored with and identifyingthe data 20 at the sink 18, as well as an amount of storage that can beemployed to store other information relevant to the ghosted file 14,including information on how to locate the sink 18, how to reconstitutethe file 14, and/or the like. Such storage in the ghosting information24 can be a variable or fixed amount, and in the latter case might belimited to 16 kilobytes or so. As may be appreciated, such ghostinginformation 24 is employed primarily by whatever construct is employedto reconstitute the file 14, and is not typically used by theaforementioned inquiring application 30 or the like, although such usemay nevertheless occur without departing from the spirit and scope ofthe present invention.

As should now be appreciated, a ghosted file 14 as ghosted may resideundisturbed on the volume 12 of the source 10 for a short time, for along time, for a very long time, or forever, at least until the volume12 no longer remains in operation. At any point in time, though, andturning now to FIG. 5, it is to be appreciated that a request to accessthe stored data 20 of the ghosted file 14 may be received from anapplication 30, a user at such application 30, etc. (step 501). Ofcourse, prior to such a request, it is also possible that one or morerequests to access the metadata 22 of such ghosted file 14 may also bereceived. For example, the aforementioned application 30 may access theset ghosted attribute in the metadata 22 of such ghosted file 14 todetermine that the file 14 is in fact ghosted, or the file system 16that controls the volume 12 may access the metadata 22 in the course ofcompiling a directory listing or the like. In the latter case, andnotably, the file system 16, the application 30, or another entity mayalso access the alternate data of the data 22 to obtain theaforementioned graphic that can be employed when displaying arepresentation of the file 14.

At any rate, in response to a request to access the stored data 20 ofthe ghosted file 14 as at step 501, the ghosted file 14 is in factreconstituted in the following manner. Preliminarily, the file 14 islocated on the volume 12 (step 503), and thereafter the ghostinginformation 24 in the metadata 22 of the file 14 is identified (step505). As may be appreciated, the data 20 of the file 14 as stored at thesink 18 is located based on such identified ghosting information 24(step 507), and such located data 20 can then be moved from the sink 18to the source 10 by way of an appropriate transport mechanism andconduit (step 511).

Of course, to store such data 20 with the file 14 at the source 10,space to be occupied by such data 20 on the volume 12 of the source 10must be allocated (step 509), with the result being that theno-longer-ghosted file 14 is no longer marked as being sparse. As maynow be appreciated, after the data 20 has in fact been moved to the file14 to reconstitute same, the metadata 22 for the no-longer-ghosted file14 is amended to show that such file 14 is not ghosted (step 513), suchas for example by resetting the ghosted attribute in such metadata 22.Also, the metadata 22 for the no-longer-ghosted file 14 is amended toremove the ghosting information 24 (step 515). Accordingly, the file 14is now in a non-ghosted form.

As should now be appreciated, there are two main scenarios which resultin creation of a ghosted file 14. In the first scenario, the file 14 iscreated directly as a ghosted file 14 on the volume 12. Such ghostcreation may be typical in a replication architecture such as that setforth above with a branch server 10 and a hub server 18. In such areplication architecture, populating each of several branch servers 10with non-ghosted copies of all files 14 from a central hub server 18would require tremendous amounts of bandwidth. Accordingly, and instead,a replication engine may decide to simply create ghosted copies of thefiles 14 on the branch servers 10. The data 20 associated with anyparticular ghosted file 14 at any particular branch server 10, then, maybe retrieved from the hub server 18 on demand as and when necessary toreconstitute such particular ghosted file 14 into a non-ghosted file 14at such particular branch server 10.

In the second scenario, the file 14 is created as a non-ghosted file 14on the volume 12 and is at some later time converted to a ghosted file14 on such volume 12. Such non-ghost creation may be typical in aspace-saving architecture such as that set forth above with a personalcomputer or the like and an alternate location 18. In such aspace-saving architecture, space on a volume 12 of the personal computeror the like is reclaimed by moving data 20 associated with files 14 thatare, for example, seldom used, to the alternate location 18, which maybe another media or to another system. As with the first scenario, thedata 20 associated with any particular ghosted file 14 at the personalcomputer or the like may be retrieved from the alternate location 18 ondemand as and when necessary to reconstitute such particular ghostedfile 14 into a non-ghosted file 14.

In one embodiment of the present invention, the operations of FIG. 5that are performed at the source 10 on behalf of a user or application30 and with respect to a ghosted file 14 are in fact performed with theaid of a ghosting filter 26, as may be seen in FIG. 2. Such operationsmay be performed transparently or with notice to the user or application30, although transparency is likely preferable to such user orapplication 30. In particular, when a user or application 30 tries toaccess the removed data 20 of a ghosted file 14, the file system 16 uponreceiving a request for such access will discover that the ghosted file14 does not contain the removed data 20 and thus return an error whichthe ghosting filter 26 will intercept. Upon such interception, then, theghosting filter 26 employs the file system 16 to obtain the ghostinginformation 24 from the metadata 22 for the ghosted file 14, and basedon the obtained ghosting information 24 such ghosting filter 26 triggersreconstituting of such ghosted file 14, as will be set forth in moredetail below, so that the request for access may ultimately be honored.

As may be appreciated, the ghosting filter 26 may be a lower-levelconstruct without much functionality and without access to networkresources such as the sink 18. In such a situation, and as shown in FIG.2, the ghosting filter 26 may interface with a higher-level constructsuch as a ghosting manager 28, where such ghosting manager 28 includesadditional ghosting functionality and direct access to network resourcessuch as the sink 18. Moreover, in such a situation, it is to beappreciated that when the ghosting filter 26 triggers suchreconstituting of such ghosted file 14, the ghosting manager 28 performsthe bulk of such reconstituting functionality for the source 10 as isshown in connection with FIG. 5, as will be set forth in more detailbelow. Such ghosting manager 28 may also perform the bulk of theghosting functionality for the source 10 as is shown in connection withFIG. 4.

Application 30 Requesting Data 20 from Ghosted File 14

Although reconstituting a ghosted file 14 has already been detailedabove in connection with FIG. 4, it is instructive to revisit suchprocess from the point of view of an application 30 or the like wishingto read some portion of the data 20 of such ghosted file 14. As may beappreciated, then, such an application 30 wishing to read such data 20from such file 14 typically achieves such a function by issuing an opencommand to the file system 16 with regard to the file 14, and thenissuing a read command to the file system 16 with regard to such openedfile 14. In one embodiment of the present invention, such application 30continues to issue such open and read commands, although such commandsare interpreted somewhat differently based on the file 14 at issue beingghosted. Such differences are transparent to and of little concern tothe application 30, although the application 30 may experience somelatency in the course of the ghosted file 14 being reconstituted, wheresuch latency likely is due in large part to moving the data 20 from thesink 18 to the source 10.

At any rate, and turning now to FIG. 6, the application 30 begins theprocess by issuing the open command with respect to a particular ghostedfile 14 on the volume 12, and such open command ultimately is receivedby the file system 16 (step 601). Notably, the application 30 in issuingsuch open command is not expected to have any knowledge that theparticular file 14 is in fact ghosted, although the application 30 couldin fact make such a determination by commanding the file system 16 toreport whether the ghosted attribute is set for the file 14.

As was noted above, the file system 16 upon receiving the open commandnotes that the ghosted file 14 does not contain the removed data 20 andthus returns an error (step 603), and the ghosting filter 26 interceptssuch returned error and perceives based thereon that the file 14 atissue is in fact in a ghosted format (step 605). Accordingly, theghosting filter 26 itself commands the file system 16 to retrieve theghosting information 24 from such ghosted file 14 and in fact receivessuch ghosting information 24 (step 607). Based on such ghostinginformation 24, then, the ghosting filter 26 creates a buffer and ahandle corresponding to the ghosted file 14, and delivers the handle tothe application 30 as the (normal) response to the open commandtherefrom (step 609).

Note that the handle for the ghosted file 14 as received by theapplication 30 represents an instance of the ghosted file 14 as opened,and is employed by the application 30 as the reference to such file 14when issuing further commands with respect to such file 14, such as forexample a read command or a close command. However, and significantly,such handle for the ghosted file 14 as created by the ghosting filter 26and as employed as a handle parameter of a command from the application30 will cause such command to be delivered directly to the ghostingfilter 26 and not to the file system 16.

Note too that the buffer for the ghosted file 14 is employed by theghosting filter 26 to store information relating to the ghosted file 14,such as for example the ghosting information 24 thereof. Thus, in thecourse of the ghosted file 14 being manipulated by the ghosting filter26, changes to such ghosting information 24 can be recorded in thebuffer and need not be written back to the file 14. Of course, after theghosting filter 26 has finished manipulating the ghosted file 14, suchrecorded changes in the buffer can be written back to the ghostinginformation 24 for the file 14 if need be.

At any rate, with the handle for the ghosted file 14 from the ghostingfilter 26, the application 30 may proceed by issuing the read commandwith respect to the particular ghosted file 14 on the volume 12, wheresuch read command has parameters including the handle, an offset intothe data 20 for the file, a read length with respect to such data 20starting from such offset, and the like. Accordingly, such read commandbased on having such handle as a parameter thereof ultimately isreceived by the ghosting filter 26 and not the file system 16 (step611). However, and as should be evident, the data 20 is not in factpresent in the ghosted file 14, and accordingly the ghosting filterstores the read command in a queue or the like for awaiting furtherprocessing (step 613).

Such further processing, as should be understood, primarily comprisesobtaining the data 20 for the ghosted file 14 from the sink 18. Inparticular, the ghosting filter 26 initiates reconstituting of theghosted file 14 based on the data 20 thereof at the sink 18 (step 615),where such reconstituting may occur in the following manner.

Remembering that the ghosting filter 26 may interface with the ghostingmanager 28 to perform higher-level ghosting functionality and directaccess to network resources such as the sink 18, the ghosting filter 26issues to the ghosting manager 28 a request to obtain some portion ofthe data 20 for the ghosted file 14 from the sink 18 (step 615 a), wheresuch request includes the ID from the ghosting information 24 of theghosted file 14, an offset into the data 20, and a length of data 20 tobe obtained starting from such offset. Thereafter, the ghosting manager28 in fact obtains the requested portion of such data 20 from the sink18 (step 615 b). Presumably, the ghosting manager 28 includes allnecessary functionality to so obtain, and does so in a manner that isknown or should be apparent to the relevant public and which need not beset forth herein in any detail. Accordingly, any particular method of soobtaining may be employed without departing from the spirit and scope ofthe present invention.

Note that it may be the case that in the course of the ghosting manager28 obtaining the requested portion of data 20 from the sink 18 as atstep 615 b, there may be some delay. For example, it may be the casethat such data 20 is obtained over a network, in which case there may bea network delay, perhaps on the order of a few milliseconds, a fewseconds, or even more. Thus, the ghosting manager 28, the ghostingfilter 26, and/or another entity may wish to maintain a pending datalist representing each portion of data 20 that has been requested fromthe sink 18 and not yet obtained therefrom, perhaps along with acorresponding request time. As may be appreciated, an identification ofeach requested portion of data is added to the pending data list uponbeing so requested, and is removed from such pending data list uponbeing received and stored with the ghosted file 14. With such pendingdata list, then, it may be the case that all pending requests for data20 are monitored for age, and that requests that are not satisfiedwithin a certain period of time may be timed out.

Once the ghosting manager 28 has in fact obtained such requested portionof such data 20 from the sink 18, such ghosting manager 28 delivers suchrequested portion to the ghosting filter 26 (step 615 c), and suchghosting filter 26 then writes such requested portion to an appropriatelocation of the file 14 at issue by way of appropriate command to thefile system 16 (step 615 d). Writing such requested portion to theappropriate location of the file 14 at issue is known or should beapparent to the relevant public and therefore need not be set forthherein in any detail. Accordingly, any particular method of so writingmay be employed without departing from the spirit and scope of thepresent invention.

Of course, it may be necessary to repeat steps 615 a-615 d a number oftimes until all of the data 20 for the file 14 is written thereto, andsuch steps are therefore repeated as necessary. Once all of the data 20for the file 14 is in fact written thereto, the ghosting filter 26issues commands as necessary to the file system 14 to fix the metadata22 of such now-reconstituted file 14, including removing the ghostinginformation 24 and resetting the ghosted attribute (step 617). Inaddition, the ghosting filter 26 retrieves all read commands for thefile 14 from the queue and passes such read commands to the file system16 for further processing (step 619), whereby the read command from theapplication 30 is in fact responded to with the data 20 at issue.

Note that the handle for the opened file 14 remains associated with theghosting filter 26 and continues to direct commands from the application30 in connection with such now-reconstituted file 14 to such ghostingfilter 26. Accordingly, it may be the case that the ghosting filter 26forwards such commands to the file system 16, it may be the case thatthe ghosting filtering 26 re-associates the handle with the file system16, or it may be the case that the ghosting filter commands anotherconstruct to re-associate the handle with the file system 16, or thelike.

Partial Reconstitution of Ghosted File 14

As may be appreciated, it should not be necessary to completelyreconstitute a ghosted file 14 in circumstances where less than all ofthe ghosted data 20 from the file 14 is required by a requestingapplication 30. Thus, if an application 30 can determine that only 1, 2,12, or 100 kilobytes at a particular offset of 2 gigabytes of data 20are needed, it should not be necessary to obtain the 2 gigabytes of data20 from the sink 18, but instead only the kilobytes of needed data 20 atthe particular offset. Moreover, in such a situation, by only partiallyreconstituting a ghosted file 14, the need to transmit a considerableamount of unneeded data 20 from the sink 18 to the source 10 is avoidedand bandwidth required to do so is correspondingly reduced.

By only partially reconstituting a ghosted file 14, then, an application30 is able to read only a few bytes of a file if need be withouttriggering a complete reconstitution of the file 14. Thus, and forexample, if only the first frame of a video file 14 is needed, suchfirst frame would be obtained from the data 20 for the file 14 at thesink 18, and not the entire amount of data 20 for the ghosted file 14,which could be on the order of 10 or even 100 gigabytes. Partiallyreconstituting, then, fetches only as much of the data 20 at the sink 18for a ghosted file 14 as is necessary, to satisfy a particular readrequest from an application 30, and no more.

Note that when partially reconstituting a ghosted file 14, it may or maynot be the case that the portion of data 20 obtained from the sink 18 isactually removed from such sink 18. On the one hand, such portion asobtained is stored at the source 10 and therefore need no longer bemaintained at the sink 18. However, on the other hand, it may actuallyrequire more effort to in fact delete the obtained portion from the sink18. Moreover, in at least some circumstances such obtained portionshould remain at the sink 18 for access by other sources 10, such as forexample if the sink is a hub server 18 and the source is a branch server10.

As may be appreciated, if a ghosted file 14 is in fact partiallyreconstituted, a record must be maintained to note what portions of theghosted file 14 are in fact reconstituted so that the ghosting filter 26can determine whether such portions are present in the ghosted file 14.Accordingly, and in one embodiment of the present invention, such arecord is maintained in the ghosting information 24 in the metadata 22of such ghosted file 14. In particular, for each contiguous section ofdata 20 of a ghosted file 14 that is reconstituted and thus is presentat the ghosted file 14, the ghosting information 24 includes therefor asection reference including an offset describing the beginning of thesection and a length describing a contiguous amount of such section.

Typically, the ghosting information 24 for a ghosted file 14 includingthe aforementioned records of partially reconstituted data 20 ismaintained by the ghosting filter 26. As was noted above, although suchghosting filter 26 could maintain and update such ghosting information24 directly in the metadata 22 of the ghosted file 14 by way of the filesystem 16, such maintaining and updating can interfere with otheroperations performed by the file system 16. Thus, the ghosting filter 26in the course of manipulating the ghosted file 14 instead initiallyobtains such ghosting information 24 from such metadata 22 and storessuch ghosting information 24 in a buffer created in connection with theghosted file 14 as at step 609 of FIG. 6, then maintains and updatessuch ghosting information 24 during the course of manipulating theghosted file 14, and upon concluding writes such ghosting information 24from the buffer to the metadata 22 for the ghosted file 14.

Note that on occasion the partial reconstituting of a ghosted file 14may be interrupted, for example by a loss of power or networkconnectivity at the source 12. Likewise, it may also be the case thatthe buffer for the ghosted file 14 is lost, especially in the case of aloss of power and where the buffer is maintained in a volatile RAM orthe like. In such a situation, the ghosting information 24 as maintainedand updated is lost and not written from the buffer to the metadata 22for the file 14, and in effect not only is such ghosting information 24lost, but all partially reconstituted data 20 associated therewith isalso lost even though physically present at the source 10, especiallyinasmuch as such data 20 cannot be located without such ghostinginformation 24. Accordingly, in one embodiment of the present invention,such ghosting information 24 as maintained and updated is periodicallywritten from the buffer to the metadata 22 for the file 14, such as forexample once every minute or so. Thus, at most, only a minute or so ofsuch ghosting information 24 and the partially reconstituted data 20associated therewith can be lost due to a loss of the buffer.

In one embodiment of the present invention, a ghosted file 14 is onlypartially reconstituted based on an appropriate command from theapplication 30 requesting data 20 from such ghosted file 14.Accordingly, such application 30 must first check with the file system16 to determine whether the ghosted attribute is set for a particularfile 14 to determine whether such file 14 is in fact ghosted, and if sothe application 30 may then by appropriate command request a partialreconstitution of a portion or portions of the data 20 for such ghostedfile 14.

In view of partial reconstitution, then, and in one embodiment of thepresent invention, the ghosting filter 26 in response to a read commandwith regard to ghosted file 14 such as that given at step 611 of FIG. 6responds thereto by first reviewing any section references in theghosting information 24 for such ghosted file 14 as stored in acorresponding buffer, and then determines from the section referenceswhether the requested data 20 or a portion thereof is already present inthe ghosted file 14 at the source 10. If all of the requested data 20 isin fact present, such data 20 is read from the ghosted file 14 at thesource 10 without need for obtaining such data 20 from the sink 18. Ifonly a portion of the requested data 20 is present, such present portionof the data 20 is read from the ghosted file 14 at the source 10 withoutneed for obtaining such data 20 from the sink 18 and the remainder ofthe data 20 is obtained from the sink 18 and then read as above. If noneof the requested data 20 is present, all of such data 20 is obtainedfrom the sink 18 and then read as above.

To summarize, then, and turning now to FIG. 7, in response to a commandfrom an application 30 to open a ghosted file 14 from which partialreconstituting may be required (step 701), the ghosting filter 26 againcreates a buffer and a handle corresponding to the ghosted file 14, anddelivers the handle to the application 30 as the (normal) response tothe open command therefrom as at step 609 (step 703). In addition, theghosting filter 26 initially obtains the ghosting information 24 fromthe metadata 22 for the ghosted file 14 and stores such ghostinginformation 24 in the created buffer (step 705).

As before, with the handle for the ghosted file 14 from the ghostingfilter 26, the application 30 issues a read command for a portion of thedata 20 of the particular ghosted file 14 on the volume 12, where suchread command includes the handle, and defines the offset and length ofthe portion as at step 611 (step 707). Here, in response to the readcommand, the ghosting filter 26 determines based on the ghostinginformation 24 in the buffer for the ghosted file 14 whether suchportion as defined is at least partially already present in the ghostedfile 14 as resident at the source 10 (step 709). Making such adetermination is known or should be apparent to the relevant public andtherefore need not be set forth herein in any detail. Accordingly, andmethod of making such determination may be employed without departingfrom the spirit and scope of the present invention.

Again, if all of the portion of the data 20 is in fact present, suchportion such data need not be obtained from the sink 18 (step 711 a). Ifonly a part of the portion of the data 20 is present, the remainder ofthe portion of the data 20 is obtained from the sink 18 (step 711 b). Ifnone of the portion of the data 20 is present, all of such portion ofthe data 20 is obtained from the sink 18 (step 711 c). Significantly,upon obtaining any part of the data 20 from the sink 18 as at steps 711b or 711 c, the ghosting filter 26 updates the ghosting information 24for the ghosted file 14 as stored in the buffer to appropriately reflectthat the part of the data 20 is now resident with and reconstituted tothe ghosted file 14 (step 713). At any rate, with the requested portionof the data 20 now resident at the source 10, such requested portion nowmay in fact be read by the application 30 as at step 619 (step 715).

As was set forth above, the ghosting filter 26 periodically writes theghosting information 24 in its most current form from the buffer to themetadata 22 for the file 14 so that such ghosting information 24 and thepartially reconstituted data 20 associated therewith is not wholly lostin the event of a loss of the buffer (step 717). In addition, once theghosted file 14 is closed, such as for example at the command of theapplication 30 (step 719), the ghosting filter 26 closes out the bufferby writing the ghosting information 24 in its most current form fromsuch buffer to the metadata 22 for the file 14 (step 721), presuming ofcourse that the ghosted file 14 has not been fully reconstituted. Thus,such ghosting information 24 in its most current form may again beretrieved at some later time as at step 705.

Fast Reading of Partially Reconstituted Ghosted File 14

An application 30 in issuing a read command with respect to a ghostedfile 14 does not take into consideration the state of the ghosted file14, and in particular does not take into consideration whether theghosted file 14 has already been partially reconstituted and/or isalready in the process of being partially reconstituted. That is, andturning now to FIG. 8, it may be the case that a particular read commandwith respect to a ghosted file 14 specifies a particular portion of thedata 20, and that such particular portion corresponds to a first segmentcomprising data 20 that is already present in the ghosted file 14, asecond segment comprising data 20 that is pending to be copied from thesink 18 to the ghosted file 14, and a third segment comprising data 20that is not present in the ghosted file 14 but is instead stored at thesink 18 only.

As may be appreciated, if such a read command were to be processed suchthat all of the particular portion of the data 20 were copied from thesink 18 to the ghosted file 14, such processing would be duplicative andwasteful with regard to at least the first and second segmentscorresponding to such particular portion. In particular, and as may alsobe appreciated, copying the data 20 corresponding to the first segmentis unnecessary inasmuch as such data 20 is already present in theghosted file 14, and copying the data 20 corresponding to the secondsegment is unnecessary inasmuch as such data 20 is already pending to becopied from the sink 18 to the ghosted file 14. In point of fact, onlythe data 20 corresponding to the third segment need be copied from thesink 18 to the ghosted file 14 inasmuch as such data 20 is not presentin the ghosted file 14 and has not already been requested to be copiedto such ghosted file 14.

In one embodiment of the present invention, then, the ghosting filter 26processes such a read command for a particular portion of data 20 byfirst identifying with regard to such particular portion of data 20corresponding segments in the ghosted file 14 that are already present(i.e., first segments), corresponding segments in the ghosted file 14that are already pending (i.e., second segments), and correspondingsegments in the ghosted file 14 that are not present or pending (i.e.,third segments), and then in fact obtaining only such third segments. Ineffect, then, the ghosting filter 26 strips out from the read commandall segments that need not in fact be read from the sink 18. Byperforming such a stripping action, the read command is handled fasterin that such ‘fast read’ obtains only that data 20 from the sink that infact is needed and not data 20 that is already present in the ghostedfile 14 or that is pending. Thus, the fast read as performed by theghosting filter 26 results in a faster response to the read command fromthe application 30, and concomitantly reduces the amount of bandwidthnecessary for such read command.

In one embodiment of the present invention, the ghosting filter 26identifies each first segment in the ghosted file 14 that is alreadypresent with reference to the ghosting information 24 maintained forsuch ghosted file 14. As was set forth above, such ghosting information24 may be referred to as located in the metadata 22 for such ghostedfile 14, or may be referred to as located in a buffer corresponding tosuch ghosted file 14. Likewise, in one embodiment of the presentinvention, the ghosting filter 26 identifies each second segment in theghosted file 14 that is already pending with reference to information inthe pending data list disclosed above in connection with step 615 b ofFIG. 6. Identifying such first and second segments with reference tosuch ghosting information 24 and pending data list should be known orapparent to the relevant public and therefore need not be set forthherein in any detail. Accordingly, such identification may be performedin any appropriate manner without departing from the spirit and scope ofthe present invention.

As may now be appreciated, once the first and second segments areidentified, the remainder is the third segment(s), and by process ofelimination, then, such third segments of the ghosted file 14 that arenot present or pending are identified. Of course, once identified, suchthird segments may in fact be requested from the sink 18 and uponreceipt copied to the ghosted file 14. Note that once requested, eachsuch third segment in effect becomes a second, pending segment. Note,too, that once received and copied to the ghosted file 14, each secondsegment in effect becomes a first, present segment. Note, finally, thatonce a segment becomes a first segment, such first segment is availableto in fact be read by the application 30 in response to the read commandtherefrom.

In at least some circumstances, it may be appreciated that although anapplication 30 has not issued a read command for particular data 20 in aghosted file 14, the ghost filter 26 nevertheless should move such data20 from the sink 18 to the ghosted file 14 in anticipation of such aread command, especially if the ghosting filter 14 is not otherwiseoccupied. As but one example, when streaming content such as a ghostedaudio file 14, a ghosted video file 14, or a ghosted multimedia file 14,it is reasonable to anticipate that a read command for data 20 at aparticular time period T0 will be followed by a read command for data 20at a following time period T1. In such a situation, the ghosting filter26 if not otherwise occupied can avail itself of the opportunity toobtain such data 20 for the time period T1 from the sink 18 even withouta specific read command from the application 30. Of course, in doing so,the ghosting filter 26 may perform such action as a fast read in themanner set forth above.

To summarize, then, and turning now to FIG. 9, the ghosting filter 26performs a fast read in response to a read command from an application30 in the following manner. Preliminarily, the read command is indeedreceived from the application 30, where such read command specifying aportion or range of the data 30 to be read from a ghosted file 14 (step901). Typically, and again, such range is expressed as an offset and alength with respect to the data 30.

Thereafter, the ghosting filter 26 first identifies within such range ofdata 20 corresponding first segments of data 20 in the ghosted file 14that are already present (step 903). Again, such identification isperformed with reference to the ghosting information 24 maintained forsuch ghosted file 14, either in the metadata 22 for such ghosted file 14or in a buffer corresponding to such ghosted file 14. If the identifiedfirst segments comprise all of the requested range of data 20, theghosted file 14 has already been reconstituted to the extent necessaryto satisfy the read command, and such read command may thus be completedwithout any further copying of data 20 from the sink 18 and withoutwaiting for any pending data 20 (step 905).

However, if the identified first segments do not comprise all of therequested range of data 20, the ghosted file 14 must indeed bereconstituted to the extent necessary to satisfy the read command, atleast based on pending data 20, and accordingly, the ghosting filter 26computes a first set comprising the range of each segment of therequested range of data 20 that is not a first segment (step 907).Thereafter, and similar to before, the ghosting filter 26 identifieswithin the first set corresponding second segments of data 20 in theghosted file 14 that are pending (step 909). Again, such identificationis performed with reference to the pending data list. If the identifiedsecond segments comprise all of the first set, the ghosted file 14 ispending to be reconstituted to the extent necessary to satisfy the readcommand, and such read command may be completed when all pending data 20is in fact copied to the ghosted file 14 (step 911).

As should now be appreciated, though, if the identified second segmentsdo not comprise all of the first set, the ghosted file 14 must indeed bereconstituted to the extent necessary to satisfy the read command, basedon data 20 to be copied from the sink 18, and accordingly, the ghostingfilter 26 computes a second set comprising the range of each segment ofthe requested range of data 20 that is not a first segment and not asecond segment (step 913), i.e., the third segments. Thereafter, theghosting filter 26 requests the second set/third segments be copied fromthe sink 18 to the ghosted file 14 (step 915).

In the course of time, and again, each third segment as requested becomea second, pending segment, and each second segment as in fact copiedbecomes a first, present segment, until all copying is complete (step917). Thereafter, the requested range of the read command is availableto in fact be read by the application 30 in response to such readcommand (step 919).

Re-Ghosting/Ghosting a File 14

As may be appreciated, a ghosted file 14 that has been reconstituted orpartially reconstituted can at some point be re-ghosted, for example ifadditional space is required at the source 10. In addition, and as mayalso be appreciated, a file 14 that has never been ghosted likewise canat some point be ghosted for similar reasons.

In particular, once a ghosted file 14 has been partially or fullyreconstituted or once a file 14 is installed at the source 10, such file14 continues to reside at the source 10 in such form unless such file 14is re-ghosted or ghosted (hereinafter, ‘re-ghosted’). Such re-ghostingis triggered by some event, which may for example be a determination ofa need for space at the source 10, a determination that the file 14 hasnot been accessed for some period of time, etc. Moreover, by re-ghostinga file 14, especially in the context of a branch server 10 and a hubserver 18, any changes to the file 14 at the source 10 or the sink 18may be replicated to the sink 18 or the source 10, respectively, to keepthe file 14 up-to-date.

In one embodiment of the present invention, then, re-ghosting at thesource 10 is performed according to a re-ghosting algorithm which takesinto account factors such as last access time, remaining free space onthe volume, frequency of access to the data, modifications to the file14 that take place elsewhere, and/or the like. The re-ghosting algorithmin considering such factors should have as a goal reducing thelikelihood of a ghosted file 14 being reconstituted again shortly afterbeing re-ghosted. Thus, bandwidth necessary for such ghosting andre-ghosting may be minimized if not reduced. Typically, although notnecessarily, the re-ghosting algorithm is performed by an application 30at the source 10 or by the ghosting manager 28 at the source 10,although other entities may perform such re-ghosting algorithm withoutdeparting from the spirit and scope of the present invention.

The re-ghosting algorithm as employed to re-ghost files 14 at a source10 may be defined at least partially based on configurable policy. Inparticular, such re-ghosting algorithm may be triggered at leastpartially based on configurable policy, and such re-ghosting algorithmmay decide to re-ghost a particular file 14 at least partially based onconfigurable policy. In each instance, such policy may be configurableby a user of the source 10, by an administrator of the source 10, or thelike.

Possible triggering parameters employed in connection with a re-ghostingalgorithm include but are not limited to:

-   -   a periodic trigger that is activated when a predetermined period        of time has elapsed;    -   a space trigger that is activated when free space at the volume        12 at the source 10 falls below a certain amount or when used        space at the volume 12 at the source 10 exceeds a certain        amount;    -   a periodic-space trigger that is activated when a predetermined        period of time has elapsed, but only if free space at the source        10 falls below a certain amount or used space at the source        exceeds a certain amount;    -   a full-volume trigger that is activated when the ghosting filter        26 notes a full-volume error returned by the file system 16 to        signify that the volume 12 has no remaining free space;    -   a bytes-downloaded trigger that is activated when a        predetermined number of bytes is downloaded to the source 10;        and    -   a manual trigger that may be activated by a user, an        administrator, or the like.        Of course, triggering the re-ghosting algorithm may occur based        on a single one of such triggers or a combination of such        triggers.

Possible selection factors for determining whether to re-ghost aparticular file 14 include but are not limited to:

-   -   the last time the file 14 was accessed, whereby a file 14 with        an older access time may be preferentially re-ghosted;    -   the download time associated with the file 14, whereby a file 14        with an older download time may be preferentially re-ghosted;    -   the file size for the file 14, whereby a larger file 14 may be        preferentially re-ghosted;    -   the file type for the file 14, whereby a file 14 with a        particular extension may be preferentially re-ghosted    -   the file attributes for the file 14, whereby, for example, a        file 14 that is a system file is not re-ghosted, but a file 14        that is hidden is re-ghosted;    -   whether multiple similar files 14 are present, whereby files 14        that are deemed similar may be preferentially re-ghosted;    -   whether the file 14 has been modified at the sink 18, whereby        such a file 14 is re-ghosted to remove data 20 therein that may        be deemed out-of-date;    -   whether the file 14 has been modified at or created at the        source 10, whereby such a file 14 may either be not selected to        be re-ghosted so as to preserve such modifications/creation or        may be selected to be re-ghosted to copy such        modifications/creation to the sink 18;    -   frequency of access to the file 14 over a particular time        period;    -   number of accesses to the file 14 over a particular time period;    -   frequency of access/number of accesses/last access time of other        related files 14, such as for example in the same folder, in the        same content set, etc.;    -   external input of specific files or types of files to be        preferentially re-ghosted, such as for example by way of a list,        a method, an XML file, etc.        Of course, the re-ghosting algorithm may employ a single        selection factor or a combination of such selection factors.

Note that files 14 selected for re-ghosting may in fact be re-ghosted,or may instead merely be designated as candidates for preferentialre-ghosting. In the latter case in particular, re-ghosting may beperformed only until a particular stop trigger has been reached. As maybe appreciated, such a stop trigger may correspond to the triggeringparameter that initiated a re-ghosting session, may correspond tocriteria for selecting candidate files 14 for re-ghosting, or may beanother trigger without departing from the spirit and scope of thepresent invention.

Note, too, that in the case where re-ghosting is performed only untilsome stop trigger is reached, it may be advisable depending on theselection criteria for files 14 that are candidates to be re-ghosted tosmooth out re-ghosting across such files 14. For example, if candidatefiles 14 are selected based on having a last access date of at least twoweeks at the source 10, it may be advisable to first re-ghost all ofsuch candidates with a last access date greater than two months, then ifnecessary to re-ghost all of such candidates with a last access dategreater than one months, then if necessary three weeks, etc. until thestop trigger is activated. Likewise, if candidate files 14 are selectedbased on having a minimum file size of 10 megabytes, it may be advisableto first re-ghost all of such candidates with a file size greater than 1gigabyte, then if necessary to re-ghost all of such candidates with afile size greater than 100 megabytes, then if necessary 50 megabytes,etc. until the stop trigger is activated. As may be appreciated, ineither scenario, some processing is required to create a list of thecandidate files 14, to sort the list, to generate one or more rounds ofre-ghosting, and the like.

Note, also, that re-ghosting may be employed to maintain consistency offiles 14 at the source 10 and at the sink 18, especially in thesituation where different versions of the file 14 may be located in bothplaces. Thus, if the file 14 is reconstituted at the source 10 and thedata 20 thereof is modified but the corresponding data 20 at the sink 18is not modified, re-ghosting may be performed to replace the data 20 atthe sink 18 with the data at the source 10. Likewise, if the data 20 ofthe file 14 at the source 10 is not modified but the corresponding data20 at the sink 18 is modified, re-ghosting may be performed only todelete such data 20 at the source 10, with the expectation that a laterreconstitution of such file will copy the data 20 at the sink 18 to thesource 10. Of course, if the data 20 of the file 14 at the source 10 ismodified and the corresponding data 20 at the sink 18 is also modified,a conflict exists, and accordingly appropriate conflict rules may haveto be consulted to determine whether and if so how to re-ghost the file14.

To summarize, then, and turning now to FIG. 10, re-ghosting of files 14on a volume 12 may be performed according to a re-ghosting algorithm,where such re-ghosting algorithm is performed by some entity in thefollowing manner. Preliminarily, the re-ghosting algorithm is triggeredby some event (step 1001), where such trigger may be internallygenerated by the entity or externally received by such entity. Upon suchtriggering, then, the re-ghosting algorithm selects files 14 from amongthe fully and partially reconstituted files 14 and the never-ghostedfiles 14 based on some selection criteria (step 1003).

At this point, the re-ghosting algorithm may merely commence byre-ghosting the selected files 14 (step 1005), or may instead considerthe selected files 14 to be candidate files 14 for possible re-ghosting(step 1007). In the latter case, the candidate files 14 are thenselected for re-ghosting in one or more rounds until a stop trigger isactivated. In particular, for each round, a group of the candidate files14 is selected (step 1009), such selected group is re-ghosted (step1011), and a determination is made of whether the stop trigger has beenactivated. If so, the process ends (step 1013). If not, the processcontinues by returning to select another group as at step 1009 (step1015).

Generic Ghosting

As thus far set forth herein, all files 14 that are ghosted at aparticular source 10 are in particular ghosted such that the data 20thereof resides at a single sink 18. However, and as may be appreciated,it could be the case that that the data 20 of each ghosted file 14 at aparticular source 10 may reside at any of a plurality of such sinks 18,as is shown in FIG. 11.

In particular, and as may be appreciated, the ghosting filter 26 of thepresent invention need not necessarily be restricted to working with asingle ghosting manager 28 to ghost data 20 from a file 14 to a singlesink 18. Instead, the ghost filter can in fact work with a plurality ofghosting managers 28, wherein each such ghosting manager 28 performsghosting functions with regard to a particular one of a plurality ofsinks 18.

In the context of a branch server 10 and a hub server 18, for example, aparticular branch server 10 can interface with multiple hub servers 18.Thus, it may be that one hub server 18 interfaced with the particularbranch server 10 has data 20 from a first source while another hubserver 18 interfaced with such particular branch server 10 has data 20from a second source. Likewise, in the context of a computing device 10and an alternate location 18, for example, a particular computing device10 can store data 20 at multiple alternate locations. Thus, it may bethat one alternate location 18 for a particular computing device 10 isdesignated as storing data 20 from files 14 of a first particular type,while another alternate location 18 for the particular computing device10 is designated as storing data 20 from files 14 of a second particulartype. Of course, the number of sinks 18 employed and the criteria fordividing data 20 as between such sinks 18 may be any appropriate numberand criteria without departing from the spirit and scope of the presentinvention.

At any rate, if multiple sinks 18 are to be employed with a particularsource 18, a mechanism is required to identify which sink 18 has data 20from a particular ghosted file 14 of such source 10, and to identifywhich corresponding ghosting manager 28 must be employed to access suchsink 18. Accordingly, and in one embodiment of the present invention,such an identification is maintained in the ghosting information 24associated with such particular file 14, as is shown in FIG. 11.

As a result, the ghosting filter 26 upon encountering any particularghosted file 14 and reading the ghosting information 24 therefrom canobtain from such ghosting information 24 the identification of theghosting manager 28 to be employed in connection with such particularghosted file 14, and based thereon can communicate with such identifiedghosting manager 28 to access such sink 18 as appropriate. In effect,then, the ghosting filter 26 is generic to all of the ghosting managers28, and the identified ghosting manager 28 controls or ‘owns’ suchparticular ghosted file 14 inasmuch as the ghosting filter 26 should notever communicate with any other ghosting manager 28 in connection withsuch particular ghosted file 14.

Presumably, each ghosting manager 28 includes all functionality andinformation necessary to communicate with the sink 18 correspondingthereto, such that the ghosting filter 26 need not be concerned withsuch matters. To the ghosting filter 26, then, communicating with theidentified ghosting manager 28 in connection with a particular ghostedfile 14 is all that is required to access the corresponding sink 18, andsuch ghosting filter 26 need not in fact be concerned with how suchidentified ghosting manager 28 communicates with such corresponding sink18, how such identified ghosting manager 28 locates such correspondingsink 18, or the like.

Similar to the processes shown in FIGS. 4 and 5, then, and now withreference to FIG. 12, generic ghosting and reconstituting of a file 14is performed in the following manner. Presumably, such ghosting isinitiated by a particular ghosting manager 28 having a particularidentification (ID) and is in fact performed by the ghosting filter 26,and such ghosting manager 28 thus sends an appropriate ghosting requestto the ghosting filter 26 with the ID and an identification of the file14 to be ghosted (step 1201), and thereafter the ghosting filter 26removes the data 20 from the file 14 as necessary (step 1203). Theghosting filter 26 then forwards such removed data 20 as necessary tothe requesting ghosting manager 28 by way of the ID thereof (step 1205),and such ghosting manager 28 may then forward such removed data to thecorresponding sink 18 based on whatever protocols for doing so have beenincluded with such ghosting manager 28.

Note in this regard that depending on the type of sink 18 and file 14,the ghosting manager 28 may in fact choose to not send such removed datato the sink 18, such as for example if the sink 18 replicated the file14 at issue to the source 10 in a read-only manner. Of course, in such acase it may be that the ghosting request from the ghosting manager 28informed the ghosting filter to in fact not perform step 1205 as beingunnecessary.

At any rate, and as before, the ghosting filter 26 amends the metadata22 for the now-ghosted file by setting the ‘ghosted’ attribute andadding the ghosting information 24 (step 1207). Significantly, suchghosting information should include the ID of the ghosting manager 28for later use when reconstituting the ghosted file 14. Thus, at somelater time when the ghosting manager 28 or an application 30 requestsaccess to the data 20 of such ghosted file 14, the request ultimatelyarrives at the ghosting filter 26 as was set forth above (step 1209) andsuch ghosting filter 26 locates the ghosting information 24 in themetadata 22 of the file 14 (step 1211).

Again, the data 20 of the file 14 as stored at the sink 18 is locatedbased on such identified ghosting information 24, although in thisinstance the ghosting filter 26 first locates within such ghostinginformation 24 the ID of the ghosting manager 28 responsible for suchghosted file 14 (step 1213), and with such ID communicates to thecorresponding ghosting manager 28 a request to in fact obtain such data20 from the corresponding sink 18 (step 1215). Presumably, such ghostingmanager 28 does in fact obtain such data 20 from such sink 18 andsupplies same to the ghosting filter 26 (step 1217), and such ghostingfilter then reconstitutes such data 20 to the file 14 at issue (step1219).

CONCLUSION

The programming necessary to effectuate the processes performed inconnection with the present invention is relatively straight-forward andshould be apparent to the relevant programming public. Accordingly, suchprogramming is not attached hereto. Any particular programming, then,may be employed to effectuate the present invention without departingfrom the spirit and scope thereof.

In the foregoing description, it can be seen that the present inventioncomprises a new and useful method and mechanism by which a file 14 at asource 10 such as a local volume 12, a computing device 10, or a branchserver 10 may be replicated or ghosted such that the data 20 thereof isstored at a sink 18 such as an alternate location 18 or a hub server 18,and the file 14 at the source is thus in a reduced or ghosted form thatcan be reconstituted if need be. The ghosted file 14 may be formed andreconstituted, as necessary.

It should be appreciated that changes could be made to the embodimentsdescribed above without departing from the inventive concepts thereof.In general then, it should be understood, therefore, that this inventionis not limited to the particular embodiments disclosed, but it isintended to cover modifications within the spirit and scope of thepresent invention as defined by the appended claims.

1. A method in connection with a computing device having a storagevolume, a file system managing the storage volume, and a file stored onthe storage volume by the file system and accessed by such file system,the file being defined to include data and metadata relating to the databut being in a ghosted form where the data has been moved to analternate location to free space on the volume, the metadata of theghosted file including ghosting information for being employed to atleast partially retrieve the moved data for the file from the alternatelocation, the method in response to the file system receiving an opencommand from an application on the computing device with respect to theghosted file and the file system upon receiving the open command notingthat the ghosted file does not contain the moved data and returning anerror, the method comprising a ghosting filter on the computing device:intercepting such returned error and perceiving based thereon that thefile is in fact in a ghosted form; commanding the file system toretrieve the ghosting information from the ghosted file and receivingsuch ghosting information; creating a handle corresponding to theghosted file, and delivering the handle to the application as theresponse to the open command therefrom; receiving a read command fromthe application with respect to a portion of the data of the openedghosted file, the read command having parameters including the handleand a definition of the portion including an offset into the portion ofthe data of the ghosted file and a read length with respect to suchportion of the data; initiating reconstituting of the portion of thedata of the ghosted file based on the received ghosting information andthe data of the ghosted file at the alternate location; and after theportion of the data of the ghosted file has been reconstitutedresponding to the read command with the portion of the data requested.2. The method of claim 1 wherein initiating reconstituting of theportion of the data comprises the ghosting filter: determining based onthe received ghosting information whether the portion of the data is atleast partially already present in the ghosted file; if a part of theportion of the data is present and a part of the portion of the data isnot present, initiating obtaining the part of the portion of the datanot present from the alternate location; and if none of the portion ofthe data is present, initiating obtaining all of such portion of thedata from the alternate location.
 3. The method of claim 2 wherein theghosting information includes for each contiguous section of datareconstituted to the ghosted file a section reference including anoffset describing a beginning of the section and a length describing acontiguous amount of such section, and wherein determining based on thereceived ghosting information whether the portion of the data is atleast partially already present in the ghosted file comprises referringto the section references in the ghosting information to determinewhether any section reference at least partially overlaps the portion ofthe data.
 4. The method of claim 1 further comprising the ghostingfilter updating the ghosting information for the ghosted file to reflectthat the portion of the data is now reconstituted to the ghosted file.5. The method of claim 1 further comprising the ghosting filter creatinga buffer for being employed by the ghosting filter to store informationrelating to the ghosted file, including the ghosting informationthereof, the ghosting filter periodically writing the ghostinginformation as current in the buffer to the metadata for the file sothat such ghosting information and any partially reconstituted dataassociated therewith is not wholly lost in the event of a loss of thebuffer, the ghosting filter upon the ghosted file being closed writingthe ghosting information as final in the buffer to the metadata for thefile.
 6. The method of claim 1 comprising the ghosting filter creatingand delivering a handle representing an instance of the ghosted file asopened, the handle for being employed by the application as thereference to such file when issuing further commands with respect tosuch file, and for directing such commands to the ghosting filter andnot to the file system.
 7. The method of claim 1 wherein the ghostingfilter initiating reconstituting of the portion of the data of theghosted file comprises: issuing to a ghosting manager on the computingdevice a request to obtain at least part of the portion of the data forthe ghosted file from the alternate location, whereby the ghostingmanager obtains the requested at least part of the portion of such datafrom the alternate location and delivers the obtained at least part ofthe portion of such data to the ghosting filter; and writing thedelivered at least part of the portion of such data to the file by wayof command to the file system.
 8. The method of claim 7 comprising theghosting filter issuing to the ghosting manager a plurality of requests,each request to obtain a different part of the portion of the data forthe ghosted file from the alternate location.
 9. The method of claim 7comprising the ghosting filter issuing to the ghosting manager a requestto obtain at least part of the portion of the data for the ghosted filefrom the alternate location, the request including an ID for the data atthe alternate location as obtained from the ghosting information of theghosted file, an offset into the part of the portion of the data at thealternate location, and a length of data to be obtained starting fromsuch offset.
 10. The method of claim 1 further comprising the ghostingfilter after the ghosted file has been reconstituted issuing a commandto the file system to remove the ghosting information from the metadataof the file if the file has been fully reconstituted.
 11. The method ofclaim 1 wherein the file is defined to include data including primarydata and secondary data, the primary data being larger than thesecondary data and having been moved to the alternate location to freespace on the volume, the method comprising the ghosting filterinitiating reconstituting of the portion of the data of the ghosted filebased on the primary data thereof at the alternate location to move suchprimary data from the alternate location to the file.
 12. A method inconnection with a computing device having a storage volume, a filesystem managing the storage volume, and a file stored on the storagevolume by the file system and accessed by such file system, the methodin response to the file system receiving an open command from anapplication on the computing device with respect to the ghosted file andthe file system upon receiving the open command noting that the file hasbeen ghosted in that data of the file has been moved to an alternatelocation and returning an error, the method comprising: interceptingsuch returned error and perceiving based thereon that the file is infact in a ghosted form; commanding the file system to retrieve ghostinginformation from the ghosted file and receiving such ghostinginformation; creating a handle corresponding to the ghosted file, anddelivering the handle to the application as the response to the opencommand therefrom; receiving a read command from the application withrespect to a portion of the data of the opened ghosted file, the readcommand having parameters including the handle and a definition of theportion including an offset into the portion of the data of the ghostedfile and a read length with respect to such portion of the data;initiating reconstituting of the portion of the data of the ghosted filebased on the received ghosting information and the data of the ghostedfile at the alternate location; and after the portion of the data of theghosted file has been reconstituted responding to the read command withthe portion of the data requested.
 13. The method of claim 12 whereininitiating reconstituting of the portion of the data comprises:determining based on the received ghosting information whether theportion of the data is at least partially already present in the ghostedfile; if a part of the portion of the data is present and a part of theportion of the data is not present, initiating obtaining the part of theportion of the data not present from the alternate location; and if noneof the portion of the data is present, initiating obtaining all of suchportion of the data from the alternate location.
 14. The method of claim13 wherein the ghosting information includes for each contiguous sectionof data reconstituted to the ghosted file a section reference includingan offset describing a beginning of the section and a length describinga contiguous amount of such section, and wherein determining based onthe received ghosting information whether the portion of the data is atleast partially already present in the ghosted file comprises referringto the section references in the ghosting information to determinewhether any section reference at least partially overlaps the portion ofthe data.
 15. The method of claim 12 further comprising updating theghosting information for the ghosted file to reflect that the portion ofthe data is now reconstituted to the ghosted file.
 16. The method ofclaim 12 further comprising creating a buffer for being employed tostore information relating to the ghosted file, including the ghostinginformation thereof, periodically writing the ghosting information ascurrent in the buffer to the file so that such ghosting information andany partially reconstituted data associated therewith is not wholly lostin the event of a loss of the buffer, and upon the ghosted file beingclosed writing the ghosting information as final in the buffer to thefile.
 17. The method of claim 12 wherein initiating reconstituting ofthe portion of the data of the ghosted file comprises: issuing a requestto obtain at least part of the portion of the data for the ghosted filefrom the alternate location, whereby the requested at least part of theportion of such data is obtained from the alternate location anddelivered; and writing the delivered at least part of the portion ofsuch data to the file.
 18. The method of claim 17 comprising issuing aplurality of requests, each request to obtain a different part of theportion of the data for the ghosted file from the alternate location.19. The method of claim 17 comprising issuing a request to obtain atleast part of the portion of the data for the ghosted file from thealternate location, the request including an ID for the data at thealternate location as obtained from the ghosting information of theghosted file, an offset into the part of the portion of the data at thealternate location, and a length of data to be obtained starting fromsuch offset.
 20. The method of claim 12 further comprising after theghosted file has been reconstituted issuing a command to remove theghosting information from the file if the file has been fullyreconstituted.